1. Field of the Invention
The present invention relates to a vacuum type booster device for vehicles and particularly, to a vacuum type booster device capable of making the operation stroke of an input member shorter than the operation stroke of an output member.
2. Discussion of the Related Art
Generally, a vacuum type booster device which is constructed to make the operation stroke of an input member shorter than the operation stroke of an output member has been known and is described in United States Patent Application Publication No. US2003/0121406 A1 (equivalent of Japanese Unexamined Published Patent Application No. 2003-191835) for example. As shown in FIG. 1 of the Application Publication, in the vacuum type booster device, a movable member 5 partitioning the interior of a shell 2 into a vacuum chamber A and a variable pressure chamber B is provided to be movable in the forward-backward direction, and a valve body 3 is provided on the movable member 5. The valve body 3 is provided with a valve mechanism 7 which is composed of an atmosphere valve 26 operable by an input shaft 8 for opening and closing the communication of the variable pressure chamber B with the atmosphere and a vacuum valve 25 for opening and closing the communication of the vacuum chamber A with the variable pressure chamber B. In the internal surface portion of the valve body 3, there is slidably inserted a cylindrical member 18 having formed thereon a vacuum valve seat 21 for the vacuum valve 25, and the cylindrical member 18 is urged backward by a spring 17. Through a variable pressure passage 28 radially holed in the valve body 3, a key member 13 passes to be movable by an axial clearance L in the forward-backward direction, and a valve plunger 16 and the cylindrical member 18 are engaged with the key member 13 to be movable by respective distances relative thereto in the forward-backward direction. An output shaft 11 is connected to an end of the valve body 3 through a reaction disc 15, so that a braking reaction force acting on the output shaft 11 is transmitted from the reaction disc 15 to the input shaft 8 through the valve plunger 16. The valve plunger 16 has formed thereon an atmosphere valve seat 22 for the atmosphere valve 26.
With this construction, even when the advance of the input shaft 8 causes the valve mechanism 7 to operate and hence, the valve body 3 to advance, the cylindrical member 18 and hence, the vacuum valve seat 21 and the key member 13 engaged with the cylindrical member 18 are held by being urged backward by the resilient force of the spring 17 at a retracted end where the key member 13 is abutting on a wall surface 2b of the shell 2 while the valve body 3 is advanced by the clearance L. Further, when the advance of the input shaft 8 brings the key member 13 into contact with a rear side end surface of the variable pressure passage 28 of the valve body 3, it results that the vacuum valve seat 21 is retracted by the clearance L relative to the valve body 3. As a consequence, the operation stroke of the output shaft 11 becomes longer by the clearance L than the operation stroke of the input shaft 8, so that it can be realized to absorb a play in the axial direction of a master cylinder or the like on the output shaft 11 side.
However, since the device described in the Application Publication is of the construction that the vacuum valve seat 21 is held at the retracted end while the valve body 3 is advanced by the clearance L, the atmosphere valve 26 remains opened even in the case of the braking manipulation being very slow, whereby the valve body 3 quickly advances by the clearance L until the rear end surface of the variable pressure passage 28 of the valve body 3 comes into contact with the key member 13. A shock operation like this may be felt by the driver as an uncomfortable feeling.
Further, in vacuum type booster devices, it is necessary for the vacuum valve 25 to secure a predetermined opening amount at the time of braking release manipulation in order to secure a predetermined responsiveness. However, since the device described in the Application Publication is of the construction that the key member 13 for determining the retracted end of the plunger 16 is utilized to hold the vacuum valve seat 21 at the retracted end and that during the non-braking operation of the valve mechanism, a first seat portion S1 of a valve member 24 is faced with the vacuum valve seat 21 with a slight clearance therebetween, the predetermined opening amount which the vacuum valve 25 has during the braking release manipulation becomes the opening amount which it has in the non-braking state. Because the predetermined opening amount becomes a loss stroke of the input shaft 8 at the time of braking manipulation, it makes a disadvantageous factor against a primary object which is to decrease the loss stroke of the input shaft 8.